Voice coil structure



Feb. 16, 1960 w. R. KOCH 2,925,541

VOICE COIL STRUCTURE Filed March 1, 1955 2/ Fz'y].

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A TZ'ORNE L United States Patent VOICE COIL STRUCTURE Winfield R. Koch, Marlton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Application March 1, 1955, Serial No. 491,253

3 Claims. (Cl. 317-158) This invention relates to a loudspeaker voice coil structure, and more particularly to a loudspeaker voice coil structure suitable for use with driving circuits having symmetrical properties.

Certain loudspeaker voice coil driving circuits have balanced or symmetrical properties. A driving circuit utilizing transistors, for example, may have critically balanced characteristics. Transistors of the junction type have symmetrical properties. One type of symmetrical property is the complementarp characteristic of n-p-n and p-n-p transistors. A loudspeaker voice coil may be driven in push-pull operation from transistor amplifier circuits exhibiting these symmetrical properties. An example of a symmetrical circuit for driving a loudspeaker voice coil may be found in the June 1953 issue of the Proceedings of the I.R.E., Fig. 6, page 719, in an article by G. C. Sziklai entitled Symmetrical Properties of Transistors and Their Applications.

For these symmetrical circuits to operate successfully, associated components must be balanced. This requires a voice coil winding which is electrically balanced to a high degree. Existing multiple-winding voice coils are wound in separate layers. These windings are unequal in length which may cause their resistance and mass to be accordingly unequal.

An object of this invention is to provide a loudspeaker voice coil which is electrically and mechanically balanced.

Another object is to provide a multiple-winding loudspeaker voice coil which is suitable for driving circuits having symmetrical properties.

A further object is to provide a multiple-winding loudspeaker voice coil which is suitable for push-pull operation by a symmetrical circuit which includes transistors.

In accordance with this invention a multiple-winding loudspeaker voice coil is wound in bifilar form. A pair of strands of electrically conductive wire of equal length and equal resistance are wound side by side or in juxtaposition to form a layer of a voice coil structure. Any given number of layers may be formed in this bifilar manner. The ends of the strands leading into and away from the voice coil structure may be symmetrically disposed about the axis of the voice coil structure to form a structure of balanced mass.

The novel features of the present invention will become apparent to. one skilled in the art from a reading of the following description in conjunction with the accompanying drawing in which:

Fig. l is a view in elevation and partially in cross section of a loudspeaker including a voice coil embodying this invention in one form;

Fig. 2 is an enlarged view in elevation and partially in cross section of a portion of the embodiment shown in Fig. 1; and

Fig. 3 is a cross-sectional view taken through Fig. 2 along the line 33 looking in the direction of the arrows.

In Fig. 1 is shown a loudspeaker incorporating a bifilar voice coil 12. The loudspeaker 10 includes a magnetic structure 14. This magnetic structure 14 includes a cylindrical permanent magnet 16. The magnet 16. is made of a permanent magnet material, alnico for example. The permanent magnet 16 is joined to the inner base of a U-shaped yoke 18 of ferromagnetic material, sofe iron for example. permanent magnet 16 extends within the voice coil form 21 which is made of paper, for example. A pole piece 22' also of ferromagnetic material, soft iron for example, lies across the ends of the U-shaped yoke 18. The pole piece 22 includes a central circular aperture 24 which cooperates with the permanent magnet extension 20 to form an annular air gap for the voice coil.

A frame member 26 is joined to the outer surface of pole piece 22 to provide support for the loudspeaker cone 28. The frame may be conveniently made of pressformed sheet steel, for example. Terminal boards 30 and 32 are joined to convenient portions of frame 26. These terminal boards are made of insulating material, fiber composition for example. The terminal boards 30 and 32 are secured to the frame 26 by steel rivets 34. Soldertype terminal lugs 36 and 38 are provided on the terminal board 30 to provide means for connection to voice coil leads, and solder-type lugs 40 and 42 are provided on the terminal board 32 to provide means of connection to other voice coil leads. Extension leads 44 and 48 of insulated wire are respectively soldered to lugs 36 and 38 on terminal board 30. Extension leads 50 and 52, also of insulated wire, are respectively soldered to lugs 40 and 42 of the terminal board 32. These leads provide means for connecting the voice coil to the output circuit of its driving means.

The voice coil is of bifilar construction. It is formed from a pair of strands 54 and 56 of electrically conductive wire. This wire may be, for example, No. 40 copper wire which is enamel insulated. These strands are continuously wound in a helical manner about a coil form 21. The starting portions of the voice coil winding are respectively designated as leads 54 and 56 The finishing portions of the voice coil winding are respectively designated as leads 54 and 56 Leads 54 and 54 are respectively soldered to lugs 36 and 38 of terminal board 30 and coil ends 56 and 56 are respectively soldered to lugs 42 and 40 of terminal board 32. Terminal boards 30 and 32 provide means for connecting the windings in center-tapped fashion or other forms of connection.

In Fig. 2 an enlarged view of the bifilar voice coil is shown. Coil form 21 is shown with a four layer bifilar voice coil 12 wound upon it. Strands or wires 54 and 56 are equal in length so that they have equal resistance. This balanced resistance property is required to provide the symmetrical characteristics required by transistor operated driving circuits. The wire is carefully chosen so that its resistive properties are uniform to aid in matching the impedance of one coil winding with the other. The portions of wires 54 and 56 which start the first layer in the multilayer voice coil are designated as leads 54 and 56 These ends are brought together and are respectively cemented to the paper form at points designated as 74 and 76. Wires 54 and 56 are wound side by side, or in juxtaposition, in a helix about the voice coil form to form the bottom or innermost layer of the voice coil. Since this illustrative voice coil is a four layer winding, wires 54 and 56 are wound back over the first layer also in continuous side by side relationship to form the second, third and fourth layers of the voice coil. The wires emerge or leave the outer layer of the voice coil structure and are there designated as leads 54 and 56 Leads 54 and 56 are respectively carried in side by side relationship to points designated as 84 and 86, respectively, where they are cemented to the coil form.

In Fig. 3, the symmetrical or balanced physical disposition of the coil leads is illustrated. Leads 54 and 56 A cylindrical extension 20 of are shown respectively cemented to points 74 and 76 on the coil form, and leads 54 and 56 are shown respec tively cemented to points 84 and 86 on the voice coil form.

This bifilar voice coil structure is both electrically and mechanically balanced. It is electrically balanced because the wires making up the voice coil are equal in length and uniform in resistivity. This provides a multiwinding voice coil in which each winding is equal in resistance. The leads into the coil and away from the coil are symmetrically positioned about the axis of the voice coil. This provides a voice coil of balanced mass. The leads are maintained in side by side relationship throughout the voice coil structure. This enhances the mutual coupling between the coil windings. This balanced mass and high mutual coupling provides a coil which operates relatively free of distortion.

What is claimed is:

1. A loud speaker voice coil comprising a cylindrical coil form, a pair of strands of electrically conductive wire of equal length, both of said strands being helically wound in one direction around said form to provide a layer of Single turns disposed longitudinally of said form, both of said strands being helically wound in the opposite direction along said form to provide another layer of single turnsdisposed longitudinally of said form and in juxtaposition with said first layer, the starting and finishing ends of said coils being disposed adjactnt to each other, the electrical resistance. of said strandsfbeing subtantially equal, and means "for securing the starting ends of said strands'at a pair of spaced positions on the surface of said coil form, means for securing the finishing ends of said strands at another pair of spaced positions on said coil form diametrically opposite said first mentioned positions whereby said ends are symmetrically disposed to provide a voice coil structure whose mass is balanced.

2. A loud speaker voice coil comprisig a coil form, a pair of strands of electrically conductive material wound in continuous side by side relationship in an even number of layers on the surface of said coil form, both of said strands in said pair being equally distant from said surface, the length of said strands being equal, said strands disposed to each other to form a having the same resistivity, and mean for securing the starting ends of said strands at a pair of spaced positions on said surface of said coil form, said positions being at substantially the same distance from one of the ends of said form, and means for securing the finishing ends of said strands at another pair of spaced positions on said coil form surface diametrically opposite the first mentioned positions, said other pair of positions being at substantially the same distance from said one end of said form whereby said ends are symmetrically disposed about said coil form to cause said voice coil to have balanced mass characteristics.

3. A bifilar loud speaker voice coil winding on a coil form comprising a pair of strands of electrically conductive wire of substantially equal lengths, said strands havin a substantially equal resistance and being helically wound side by side to form an even number of coaxial cylindrical layers of turns and means for securing the starting ends of said strands at a pair of spaced positions on the surface of said coil form, and means for securing the finishing ends of said strands at another pair of spaced positions on said coil form surfact diametrically opposite said first mentioned positions, said first named positions being at substantially the same distance from one of the ends of said form, said other pair of positions being at'substantially said same distance from said one end of said form whereby said ends are symmetrically structure of balance mass.

1 References Cited in the file of this patent UNITED STATES PATENTS' 634,198 Buckingham et al. Oct. 3, 1899 1,814,119 Bradbury July 14, 1931 1,868,607 Kolster July 26, 1932 1,972,319 Rypinski Sept. 4, 1934 2,178,653 Slade Nov. 7, 1939 2,316,370 7 Smith et a1. Apr. 13, 1943 2,550,891 Wald May 1, 1951 FOREIGN PATENTS 840,351 France Jan. 16, 1939 UNITED STATES PATENT OFFICE CERTIFICATE ()F CORRECTION- Patent Noo 2,925,541 February 16, 1960 Winfield R. Koch It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 5, for "sofe" read soft column 4, line 22 for "surfact" read surface Signed and sealed this 11th day of October 1960.

( SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attcsting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0a 2,925,541 February 16, 1960 Winfield R. Koch It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should readas corrected below.

Column 2 line 5, for "sofe" read soft column 4, line 22 for "surfact" read surface Signed and sealed this 11th day of October 1960. 1

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents 

